Organically Directed Bimetallic Sulfates Based on the Radii and

The Ln3+ cations with high coordination number gives a straight chain, and the Cd2+ cation with lower coordination number leads to wavy chain. Another...
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Organically Directed Bimetallic Sulfates Based on the Radii and Coordination Number: Synthesis, Structure and Characterization Yunlong Fu,* Zhiwei Xu, Jialin Ren, and Junying Yang School of Chemistry and Material Science, Shanxi Normal UniVersity, Linfen Shanxi, P.R. China, 041004

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 6 1198-1204

ReceiVed October 25, 2006; ReVised Manuscript ReceiVed April 2, 2007

ABSTRACT: A series of organically directed bimetallic sulfates with one- and two-dimensional structures have been synthesized hydrothermally. The 2D structures with formulas (C2H10N2)2[FeIII2FeII(SO4)6(H2O)4]‚2H2O, I, (C2H10N2)2[FeIII2CdII(SO4)6(H2O)4]‚ 2H2O, II, and (C2H10N2)2[FeIII2NiII(SO4)6(H2O)4]‚2H2O, III, have identical brick-wall layers constructed from infinite ferrinatritetype {Fe2(SO4)3}n chains jointed by transition metal ions M2+ (M ) Fe, Cd, Ni); the 1D structures with the compositions [C2N2H10][Fe3O(SO4)6La(H2O)6]‚3H2O, IV, [C2N2H10][Fe3O(SO4)6Ce(H2O)6]‚3H2O, V, and [C4N2H12]1.5[Fe3O(SO4)6Cd(H2O)5]‚ 2H2O, VI, possess sulfated trinuclear iron oxo clusters that are interconnected by Ln3+ or Cd2+cations to extended straight or zigzag chains. The assembly of iron sulfate, resulting from the increase of the radii and coordination numbers of second metal centers, was modulated to afford a 1D chain or 0D cluster and further extended to form distinct 2D layer or 1D cluster-containing chain. The magnetic measurements reveal the presence of ferromagnetism for I at low temperature and two paramagnetic regions for VI. Introduction In the past few years, the design and synthesis of bimetallic compounds, especially 3d-4f metal systems, have attracted increasing attention1-6 because of their fascinating structural diversity and potential applications as functional materials in magnetism,3 molecular adsorption,4 light conversion devices,5 and bimetallic catalysis.6 In terms of crystal engineering, bimetallic structures have been reported successively for molybdate,7 vanadate,8 carboxylates,9 phosphate,10 etc., displaying diverse topologies and properties. Recently, 1D, 2D, and 3D inorganic open frameworks constructed from sulfate tetrahedra and MOn (n ) 4-12) polyhedra have been extensively studied, and some of them have revealed rich topologies and interesting physical properties. However, the studies of organically directed metal sulfates have been mainly focused on the monometallic system;11 therefore, the organically directed bimetallic sulfates are still undeveloped.12 Up to now, organically directed iron sulfates showed interesting structures and magnetic properties,13 and the incorporation of a second metal should improve the topology and property, resulting in the formation of new inorganic materials. Hence, the design and synthesis of new bimetallic iron-based sulfates are attractive in the fields of crystal engineering and novel materials. In this study, the experiments were performed in a Fe3+M2+(Ln3+)-amine-H2SO4 system (M ) Mg, Mn, Fe, Co, Ni, Zn, and Cd; Ln ) La and Ce) under mild hydrothermal conditions. A series of 3d-3d, 3d-4f, and 3d-4d bimetallic sulfates, namely, 2D [C2H10N2]2[FeIII2MII(SO4)6(H2O)4]‚2H2O (M ) Fe, Cd, and Ni) with brick-wall layers, 1D [C2H10N2][Fe3O(SO4)6Ln(H2O)6]‚3H2O (Ln ) La and Ce) with straight chains, and 1D [C4H12N2]1.5[Fe3O(SO4)6Cd(H2O)5]‚2H2O with wavy chains, have been prepared. The introduction of the second metal ions with different radii and coordination numbers results in distinct extended layers and chains, and some of them reveal interesting magnetic properties. The compounds were characterized by single-crystal X-ray diffraction, powder X-ray diffraction * To whom correspondence should be addressed. Phone: +86 357 2053716. fax: +86 357 2053716. E-mail: [email protected].

Figure 1. TG curves for II and VI.

(XRD), infrared spectroscopy analysis (IR), thermogravimetric analysis (TGA), energy-dispersive X-ray analysis spectroscopy (EDX), CNH elemental analysis, and magnetic analysis. Experimental Section Measurement. Solvents and reagents were obtained from commercial sources and were used without further purification. The CNH elemental analyses were carried out on a Perkin-Elmer 240 elemental analyzer. Thermogravimetric analyses (TGA) of the samples were performed on a ZRP-2P thermal analyzer under flowing N2 at a heating rate of 10 °C min-1 from room temperature to 680 °C. The infrared (IR) spectra were recorded (450-4000 cm-1 region) on a Nicolet 5DX spectrometer using KBr pellets. Magnetic measurements were measured on the collected crystals using a PPMS-9T magnetometer at a field of 1 KOe in the temperature range of 2-300 K. The ratio of heavy atoms in the same sample was determined by energy-dispersive X-ray analysis spectroscopy (EDX). The XRD patterns were obtained on a Bruker D8X diffractometer, equipped with a graphite monochromator, which allows the use of Cu KR radiation (KR1 ) 1.5406; KR2 ) 1.5444), and the data were collected in the range of 4 e 2θ e 40° at room temperature. Synthesis and Initial Characterization. Compounds I-VI were synthesized under mild hydrothermal conditions. The organic amines employed here are ethylenediamine (en) and piperazine (pip), respectively. In a typical synthesis of I, ferric sulfate nonahydrate (0.286 g, 0.50 mmol) and iron(II) sulfate hexahydrate (0.261 g, 1.00 mmol) were

10.1021/cg0607506 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007

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Table 1. Crystal Data and Structure Refinement Parameters

empirical formula fw T (K) λ (Å) cryst syst space group cryst size (mm) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) Rint GOF on F2 R1a [I > 2σ(I)] wR2b [I > 2σ(I)] a

I

IV

VI

C4H28N4Fe3O30S6 972.18 298(2) 0.71073 triclinic P1h (No. 2) 0.18 × 0.16 × 0.15 8.818(3) 8.848(3) 10.682(3) 97.806(6) 111.902(6) 104.815(6) 722.4(4) 1 2.226 2.046 0.0588 1.105 0.0751 0.2261

C2H28N2Fe3LaO34S6 1123.04 298(2) 0.71073 orthorhombic Pbca (No. 61) 0.08 × 0.30 × 0.36 18.7407(10) 15.8130(8) 20.0723(10) 90 90 90 5948.4(5) 8 2.499 3.395 0.0342 1.019 0.0277 0.0693

C6H32N3Fe3CdO32S6 1210.58 298(2) 0.71073 monoclinic P21/n (No. 14) 0.29 × 0.21 × 0.15 9.6811(8) 19.2501(16) 17.6856(15) 90 97.7080(10) 90 3266.1(5) 4 2.291 2.453 0.0288 1.073 0.0385 0.1068

R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.

dissolved into a diluted sulfuric acid (0.06 mL of 98% H2SO4 and 6.0 mL of H2O); then a total of 0.06 mL (1.00 mmol) of en was added. Ethanol (2.0 mL) was added to generate a homogeneous gel. The final mixture was transferred into a 15 mL Teflon-lined autoclave and heated at 383 K for 2 days. After the mixture was cooled to room temperature, the dark pink crystals were filtered, washed with ethanol, and dried in air at ambient temperature with yield of about 70% based on Fe3+. The preparations of II-VI are similar to that of I, and the details are given in the Supporting Information. The energy-dispersive X-ray analysis spectroscopy (EDX) and CNH elemental analyses conformed the expected element ratios. (Detailed results are included in Supporting Information.) The infrared (IR) spectra for I and II were recorded within the 4504000 cm-1 region. The results reveal the similar spectra for two compounds (3522 (s), 3148 (s), 2665 (w), 2538 (w), 2400 (w), 2203 (w), 2005 (w), 1653 (s), 1505 (s), 1465 (s), 1407 (s), 1210 (vs), 1111 (s), 1023 (s), 826 (m), 689 (m), 591 (s), 482 (m), 443 (m)). Compounds I, II, and VI were used for powder X-ray diffraction analyses because III, IV, and V were impure (Supporting Information). The experimental and simulated powder X-ray diffraction (P-XRD) patterns are in good agreement with each other, which prove the phase purity for I, II, and VI. II and VI were used for thermogravimetric analyses (TGA). TG curves for II and VI show three- and four-step weight losses, respectively (Figure 1). For II, the weight loss between 100 and 247 °C corresponds to the loss of lattice and coordinated waters (obsd ) 9.80%, calcd ) 10.15%); the continuous weight losses between 276 and 640 °C indicate that amine and sulfate groups are released in two stages (obsd ) 68.67%, calcd ) 68.09%). For IV, the coordination and lattice water is set free between 140 and 298 °C (obsd ) 14.41%, calcd ) 14.42%). The weight losses between 298 and 424 °C correspond to the loss of amines and two sulfate groups per formula unit (obsd ) 20.19%, calcd ) 19.76%); the weight loss between 521 and 658 °C corresponds to the loss of two sulfate groups per formula unit (obsd ) 14.23%, calcd ) 14.24%). The TG curve for VI displays a shape similar to that of IV. The TG curve shows a continuous weight loss between 150 and 425 °C, corresponding to the losses of coordinated waters, uncoordinated waters, the amines, and sulfate groups in three stages (obsd ) 43.60%, calcd ) 43.78%); the weight loss between 521 and 650 °C corresponds to the loss of the remaining sulfate groups (obsd ) 14.25%, calcd ) 12.54%). The powder X-ray diffraction pattern of the sample heated after 680 °C corresponds to Fe2O3-CdO, Fe3O3(SO4)1.5-LaO(SO4)0.5, and 3Fe2O32CdO.11b-11d,17 X-ray Crystallographic Study. Suitable single crystals were subjected to crystallographic studies. The single crystals were mounted on glass fibers using cyanoacrylate. The intensity data were collected on a Bruker SMART 1000 CCD diffractometer equipped with graphite-monochromated Mo KR radiation (0.71073 Å at a tempera-

ture of 298 ( 2 K). The data were collected at 293 ( 2 K using the ω-2θ scan technique to a maximum 2θ value of 54.0°. Absorption corrections based on symmetry-equivalent reflections were applied using SADABS. The structures were solved by direct methods using SHELXS-97 and difference Fourier synthesis. The direct methods readily revealed the heavy-atom position (Fe, Cd, Ni, Ln, and S) and allowed us to locate the other non-hydrogen positions (O, C, and N) from the difference Fourier syntheses. The hydrogen atoms of amine molecules are placed theoretically. The last cycles of refinement included the atomic positions for all the atoms, anisotropic thermal parameters for all non-hydrogen atoms, and isotropic thermal parameters for all the hydrogen atoms. However, some of the hydrogen atoms for the coordinated and lattice water molecules are not included in last circle. The en molecule in V is disordered over two positions because the occupancy refined to nearly 0.5:0.5. Details of the crystal data, intensity collection, and some features of the structure refinements were reported in Table 1. The crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre with CCDC reference numbers 616068616073.

Results Crystal Structure. Structural analyses reveal that I, II, and III are extremely similar with slight differences in the crystal parameters (Table S3), while IV and V exhibit identical inorganic chains. Hence, only I, IV, and VI are taken as examples to discuss their structures in detail. Selected important bond distances are listed in Table 2. Hydrogen-bonding interactions are given in the Supporting Information (Table S2). [C2H10N2]2[FeIII2FeII(SO4)6(H2O)4]‚2H2O, I. The asymmetric unit of I contains 25 non-hydrogen atoms of which 20 belong to the inorganic layer with three iron atoms and three sulfur atoms being crystallographically distinct (Figure 2). The Fe(1) and Fe(2) atoms are octahedrally coordinated to the O atoms from six sulfate groups. The Fe(3) atom is six-coordinated by two O atoms from the sulfate groups and four O atoms from the water molecules in an octahedral geometry. Bond-valence sum (BVS) calculations14 (Fe(1) ) 3.03, Fe(2) ) 3.02, and Fe(3) ) 1.98) and the average Fe-O bond lengths indicate the oxidation state of Fe(1) and Fe(2) to be +3 and that of the Fe(3) centers to be +2 (Table 2). Three SO4 tetrahedra link two adjacent FeIIIO6 octahedra along the a-axis via shared vertexes, giving rise to an infinite ferrinatrite-type14 chain. These chains are connected to one another by the FeII(3)O2(H2O)4

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Table 2. Selected Bond Distances for I, IV, and VIa I Fe(1)-O(9) Fe(1)-O(9)#1 Fe(1)-O(2)#1 Fe(1)-O(2) Fe(1)-O(4) Fe(1)-O(4)#1 Fe(2)-O(6)#2 Fe(2)-O(6) Fe(2)-O(1) Fe(2)-O(1)#2

1.987(5) 1.987(5) 1.998(4) 1.998(4) 2.004(4) 2.004(4) 1.982(5) 1.982(5) 1.995(5) 1.995(5)

Fe(3)-O(8)#3 Fe(3)-O(12)#3 Fe(3)-O(12) Fe(2)-O(5)#2 Fe(2)-O(5) Fe(3)-O(13) Fe(3)-O(13)#3 Fe(3)-O(8) S(1)-O(3) S(1)-O(6)

La(1)-O(25) La(1)-O(21) La(1)-O(14) La(1)-O(27) La(1)-O(31) La(1)-O(10) La(1)-O(28) La(1)-O(23) La(1)-O(13) Fe(1)-O(5) Fe(1)-O(19) Fe(1)-O(9) Fe(1)-O(17) Fe(1)-O(15) Fe(1)-O(7) Fe(2)-O(5) Fe(2)-O(24)

2.450(2) 2.459(2) 2.498(2) 2.499(2) 2.517(2) 2.521(2) 2.619(3) 2.668(3) 2.727(3) 1.9379(17) 2.001(2) 2.004(2) 2.005(2) 2.015(2) 2.065(2) 1.9494(18) 1.990(2)

Fe(2)-O(18) Fe(2)-O(11) Fe(2)-O(12) Fe(2)-O(1) Fe(3)-O(5) Fe(3)-O(26) Fe(3)-O(2) Fe(3)-O(3) Fe(3)-O(6) Fe(3)-O(22) S(1)-O(4) S(1)-O(14)#12 S(1)-O(6) S(1)-O(15) S(2)-O(32) S(2)-O(31) S(2)-O(9)

Cd(1)-O(29) Cd(1)-O(30) Cd(1)-O(28) Cd(1)-O(16) Cd(1)-O(27) Cd(1)-O(23) Fe(1)-O(1) Fe(1)-O(5) Fe(1)-O(19) Fe(1)-O(20) Fe(1)-O(6) Fe(1)-O(24) Fe(2)-O(1) Fe(2)-O(9) Fe(2)-O(10) Fe(2)-O(11)

2.219(4) 2.239(4) 2.250(3) 2.256(3) 2.330(3) 2.361(3) 1.930(3) 1.977(3) 1.998(3) 2.019(3) 2.036(3) 2.076(3) 1.940(3) 1.993(3) 2.006(3) 2.027(3)

2.138(5) 2.152(5) 2.152(5) 2.006(5) 2.006(5) 2.114(5) 2.114(5) 2.138(5) 1.458(5) 1.486(5)

S(1)-O(10) S(1)-O(2) S(2)-O(7) S(2)-O(11) S(2)-O(1) S(2)-O(9) S(3)-O(15) S(3)-O(12) S(3)-O(5) S(3)-O(4)

1.449(5) 1.506(5) 1.456(5) 1.461(5) 1.477(5) 1.488(5) 1.433(5) 1.456(5) 1.482(5) 1.499(5)

S(2)-O(2) S(3)-O(8) S(3)-O(10) S(3)-O(18) S(3)-O(3) S(4)-O(30) S(4)-O(27)#12 S(4)-O(24) S(4)-O(26) S(5)-O(21)#12 S(5)-O(16) S(5)-O(19) S(5)-O(12) S(6)-O(25) S(6)-O(20) S(6)-O(11) S(6)-O(17)

1.484(2) 1.442(2) 1.443(2) 1.474(2) 1.487(2) 1.447(2) 1.455(2) 1.472(2) 1.481(2) 1.446(2) 1.457(3) 1.484(2) 1.487(2) 1.434(3) 1.453(3) 1.481(2) 1.491(2)

IV 2.001(2) 2.020(2) 2.024(2) 2.054(2) 1.947(2) 1.983(2) 1.986(2) 1.995(2) 2.009(2) 2.112(3) 1.448(2) 1.450(2) 1.481(2) 1.482(2) 1.448(2) 1.448(2) 1.473(2) VI Fe(2)-O(2) Fe(2)-O(25) Fe(3)-O(1) Fe(3)-O(15) Fe(3)-O(21) Fe(3)-O(14) Fe(3)-O(17) Fe(3)-O(26) S(1)-O(13) S(1)-O(16) S(1)-O(11) S(1)-O(14) S(2)-O(23) S(2)-O(22) S(2)-O(20) S(2)-O(21)

2.061(3) 2.069(3) 1.935(3) 1.981(3) 2.007(3) 2.018(3) 2.019(3) 2.051(3) 1.439(3) 1.458(3) 1.491(3) 1.496(3) 1.458(3) 1.462(3) 1.483(3) 1.486(3)

S(3)-O(3) S(3)-O(4) S(3)-O(2) S(3)-O(6) S(4)-O(8) S(4)-O(7) S(4)-O(9) S(4)-O(5) S(5)-O(28)#13 S(5)-O(12) S(5)-O(15) S(5)-O(10) S(6)-O(27)#13 S(6)-O(18) S(6)-O(17) S(6)-O(19)

1.453(3) 1.459(3) 1.479(3) 1.486(3) 1.437(3) 1.468(3) 1.492(3) 1.498(3) 1.449(3) 1.450(3) 1.482(3) 1.489(3) 1.440(3) 1.456(3) 1.485(3) 1.497(3)

a Symmetry transformations used to generate equivalent atoms: #1 -x, -y + 1, -z + 2; #2 -x + 1, -y + 1, -z + 2; #3 -x, -y + 1, -z + 1; #4 -x, -y, -z; #5 -x, -y, -z + 1; #6 -x + 1, -y, -z + 1; #7 -x + 1, -y + 2, -z + 1; #8 -x + 1, -y, -z; #9 -x + 1, -y + 1, -z; #10 x, y + 1, z; #11 x, y - 1, z; #12 x, -y + 3/2, z - 1/2; #13 -x + 1/2, y + 1/2, -z + 1/2.

octahedra, in such a way that each FeIIO2(H2O)4 octahedron shares two axial oxygen atoms with SO4 groups of inorganic chains, giving rise to a net sheet with brick-wall topology in the ac-plane (Figure 3). The diprotonated en molecules reside between the sheets and charge compensate the anionic inorganic layers. [C2N2H10][Fe3O(SO4)6La(H2O)6]‚3H2O, IV. The asymmetric unit of IV contains 48 non-hydrogen atoms of which 44 belong to the inorganic chain with one lanthanum atom, three iron atoms, and six sulfur atoms being crystallographically distinct (Figure 4). The La atom is nine-coordinated by six O atoms from sulfate groups and three O atoms from water molecules in a near-perfectly tricapped trigonal prismatic geometry (Table 2). The Fe atom is six-coordinated by four O atoms from the sulfate groups, one µ3-O atom, and one coordinated water molecule, in other words, three Fe atoms form a unique Fe3O unit via a µ3-O atom (Table 2). The Fe3O unit is slightly distorted (Fe-Fe distances ) 3.393(1), 3.353(0), and 3.363(1) Å), and each pair of Fe atoms is bridged by two sulfate groups, one above and one below the Fe3O plane. Each LaO9 polyhedron symmetrically shares six vertices with adjacent

[Fe3O(SO4)6(H2O)3]5- clusters, generating a straight chain along the c-axis (Figure 6a). The chains are held together into a 3D assembly through the interchain O-H‚‚‚O hydrogen bond network along the b- and a-axes (Figure 7a). The diprotonated en molecules and lattice water molecules reside in the voids and interact with inorganic chains via hydrogen bonds to stabilize the 3D structure. [C4N2H12]1.5[Fe3O(SO4)6Cd(H2O)5]‚2H2O, VI. Of the 51 asymmetric non-hydrogen atoms in VI, 42 belong to the inorganic chain with one cadmium atom, three iron atoms, and six sulfur atoms being crystallographically distinct (Figure 5). Compound VI contains a sulfated iron oxo cluster identical to that of IV, with slight differences in the different Fe-Fe distances (3.370(2), 3.352(2), and 3.331(1) Å). Each Cd atom is six-coordinated to O atoms from four sulfate groups and two water molecules. The Cd-O bond distances are in the range of 2.219(4)-2.361(3) Å, and the cis-O-Cd-O bond angles range from 78.49(13) to 96.78(13)°, indicating a distorted octahedral geometry (Table 2). The distorted CdO4(H2O)2 octahedron links adjacent clusters via sharing four equatorial vertices with SO4 tetrahedra, giving rise to a wavelike chain along the b-axis

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Figure 2. Asymmetric unit for I. Here, Fe(1) and Fe(3) are in the +3 oxidation state and Fe(3) is in the +2 state. Symmetry codes: (A) -x, -y + 1, -z + 2; (B) -x + 1, -y + 1, -z + 2; (C) -x, -y + 1, -z + 1. Figure 5. Asymmetric unit for IV. Symmetry codes: (A) -x + 1/2, y + 1/2, -z + 1/2.

Figure 3. (a) Polyhedral view of the featured inorganic layer in I-III; the ferrinatrite-type chains propagating along the a-axis are interconnected by MO6 octahedra to form a net sheet (dotted-line polyhedra, FeIIIO6; crosslike polyhedra, MO6 ) FeIIO6, CdIIO6, NiIIO6; tetrahedra, SO4). (b) Representation of the brick-wall layer in ac plane.

Figure 6. Two unique chains that consist of identical sulfated trinuclear iron oxo clusters µ3-O[Fe(SO4)2(H2O)]3 and different linkers: (a) LnO9 polyhedra in IV and (b) CdO6 octahedra in VI, respectively.

Discussion

Figure 4. Asymmetric unit for IV. Symmetry codes: (A) x, -y + 3/2, z - 1/2.

(Figure 6b). These chains are held together along the [101] direction by interchain O-H‚‚‚O hydrogen bonds (Figure 7b), giving rise to a layered arrangement. These pseudolayers are stacked one over the other along the a-axis, with the lattice water molecules and diprotonated pip molecules residing in the interlamellar space. The hydrogen-bonding interactions between the amines and the oxygens of the framework stabilize the 3D assembly.

Synthesis. Under the hydro(solvo)thermal condition, the Fe3+ ion was liable to be reduced to Fe2+,12 and compound I was first found as a byproduct in the Fe3+-H2SO4-en system.16,17 The improved experiment involving the introduction of the Fe2+ ion into the starting mixture yielded I as a pure phase. Compounds II and III were obtained from the Fe3+-M2+H2SO4-en system, with M ) Cd and Ni, respectively. As extended experiments, the other transition metal ions (Zn2+, Co2+, and Mn2+) and an alkali earth metal ion (Mg2+) were also employed and resulted in the crystalline products with similar cell parameters from the single-crystal XRD analyses (Table S3). For these bimetallic Fe3+-M2+ (M ) Mg, Mn, Fe, Co, Ni, Zn, and Cd) compounds, the powder XRD studies displayed similar peak positions (Figure S6), and the energydispersive X-ray analyses (EDX) confirmed a consistent ratio of Fe3+/M2+/S ) 2:1:6, implying structural similarity. However, the introduction of rare earth metal La3+ and Ce3+ with large radii, unexpectedly, gave rise to IV and V, and in the Fe3+Cd2+-pip system, VI instead of II was produced, implying a certain influences of amines. Structural Discussion. As mentioned earlier, iron sulfate has a strong tendency to form 1D chain.12,15,16 When the second metal cations with different coordination abilities were intro-

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Figure 8. χm-T product and hysteresis loop of the magnetization (inset) for I.

Figure 7. Dotted view of the possible hydrogen-bonding interactions including the O-H‚‚‚O hydrogen bonds in (a) IV and (b) VI and the N-H‚‚‚O hydrogen bonds in (a) IV.

duced into iron-sulfate system, some known structural motifs were preferably constructed and interlinked to novel extended structures. As the second metal cations with small radii were introduced into Fe3+-en-H2SO4 system, the ferrinatrite-type chains were formed and interlinked into an infinite layer. Interestingly, such a ferrinatrite-type iron sulfate chain had been isolated in the previous experiment,17 and the extended Fe-M bimetallic layers were also prepared by using this preformed crystal as source. Although diverse iron sulfates chains, such as tancoite,12 ferrinatrite,17 butlerite,12 kro¨hnkite,17 and copiapite-type,17 etc., have been separated, only the ferrinatrite-type chain was extended into 2D layers by binding to second metal centers. For these ferrinatrite-type chains completed by sulfate groups, their high negative charge of itself may be an important factor in allowing the chains to bond easily with these transition metal cations, even alkali earth Mg2+ cations. It is believed that such layered topology with a consistent formula [FeIII2MII(SO4)6(H2O)4]4- is a rather stable structure produced from this bimetallic system, and the M2+ site can be displaced by several metal ions. Compared with the 3d-shell transition metal ions, the 4f-shell Ln3+ ions possess higher coordination numbers and large radii. Subsequently, the sulfated trinuclear iron oxo clusters are formed and symmetrically interconnected by the Ln3+ cation with six-bridging mode to form a straight extended 1D chain. Although a few parameters should influence the formation of the [Fe3O(SO4)6(H2O)3]5- cluster, actually, it is believable to say that sulfate anions preferably connect Fe cations (bond valence ) 0.5) instead of Ln3+ because the Ln-O distance is greater than ∼2.5 Å (Table 2). Thus, the formation of the cluster could be favored. The Ln3+ cation acts as linker between the cluster. Relatively, the 4d-shell Cd2+ cation with less

coordination number and large radius not only gives a bimetallic layer in the Fe3+-Cd2+-en system but also leads to a [Fe3O(SO4)6(H2O)3]5- cluster-containing chain in the Fe3+Cd2+-pip system. Different from the Fe-Ln straight chain, the Fe-Cd structure displays a wavy chain resulting from the 4-bridging mode of CdO6 octahedra. To the best of our knowledge, the discrete sulfated trinuclear iron oxo clusters have emerged in their sodium, potassium, and rubidium salt forms,18 in which the alkali metal atoms are distributed around the isolated anionic cluster as the counterions. Directed by the diprotonated amines, the different coordination numbers of second metals play an important role for construction of the different Fe3O cluster-containing linear topologies. In contrast to the Fe3O cluster-containing MOFs (metal-organic frameworks),19 the extended inorganic structure based on the trinuclear iron oxo cluster has not been explored. Because of the hydrolysis of the aqueous ferric ions, the separation of the isolated iron oxo cluster is a notable subject; furthermore, the preparation of extended structures based on such clusters offers a possibility for the design of a polymeric solid-state material of a given composition and a predictable property. Hence, the study of the bimetallic inorganic compound will be a facile route to realize novel extended structure with desirable properties. Among these structures, a great deal of O-H‚‚‚O hydrogen bonds involving the coordinated water molecules lead to an interesting phenomenon: in addition to the previously mentioned N-H‚‚‚O hydrogen-bonding interactions, many intrachain, interchain, and intralayer hydrogen bonds are also observed between the coordinated water molecules and the nearby sulfate groups (Figure 7b). Such intrachain and intralayer hydrogen bonds are a little rare11 in the metal sulfates and take important roles in stabilization of the crystal structure. Hence, hydrogenbonding interactions are not only an important part in an aquabiology system20 but also are an interesting subject involving the crystal engineering.21 Magnetic properties. The presence of mixed-valent iron sulfate in I and the trinuclear iron oxo cluster in VI provide a valuable opportunity for the magnetic properties study. The temperature dependences of the magnetic susceptibilities of I and VI were measured in a magnetic field of 1 KOe and fitted using the Curie-Weiss equation. The magnetic susceptibility, χm, of I shows a smooth increase with decreasing temperature from room temperature and a very sharp increase at about 9 K (Figure 8), and it reached a maximum (10.85 emu mol-1) at 2 K, suggesting ferromagnetic behavior (Tc ) 5K). Support for the ferromagnetism also comes from the field dependence of the magnetization at 2 K, and a

Organically Directed Bimetallic Sulfates

Crystal Growth & Design, Vol. 7, No. 6, 2007 1203 Fe-Mg, Fe-Mn, and Fe-Co compounds, EDX and CHN element analysis results, IR spectra, PXRD patterns, comparison of elemental and simulated PXRD pattern for I, II, IV, and VI, and curves of χm versus T for I. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 9. Thermal dependence of the χm and χm-1 (inset) plots for VI.

small hysteresis loop was observed (inset of Figure 8). In addition, the linear χm-1-T curve above Tc indicates paramagnetism, and the value of θ ) -11.186 K suggests antiferromagnetic interactions between Fe neighbors (Figure S7). The value of the effective magnetic moment (µeff) for I obtained from the Curie-Weiss region is 9.8 µB, which corresponds to a spin-only value predicted for a mixed Fe3+/Fe2+ system (S ) 9/2). The temperature dependence of the molar magnetic susceptibility χm of VI increases upon cooling to 2 K, indicating the presence of the dominated paramagnetism (Figure 9). The χm-T plot reveals two different paramagnetic regions with decreasing temperature. A temporary shoulder occurred in the range of 33-80 K, implying a transformation from one magnetic exchange to another. From 300 to 89 K, the χm-1 value decreases smoothly with the value of Cm ) 10.7326 cm3 mol-1 K and θ ) -486 K. The large negative Weiss constant suggests a strong antiferromagnetic interaction between the Fe neighbors (short Fe-Fe distances of ∼3.331-3.370 Å).22 In the extremely lowtemperature region of 20-2 K, the χm-1 value decreases rapidly with a couple of value Cm ) 0.4624 cm3 mol-1 K and θ ) -4.087 K. The small negative Weiss constant can be attributed to weak antiferromagnetic exchange or a paramagnetic impurity,23 in contrast to the linear χm-1-T curve for the isolated trinuclear iron oxo cluster.17 The introduction of the Cd atoms into the iron-sulfate system made the Curie constant increase at high temperature and decrease at low temperature.18 Such a situation is rather rare and has not been accounted in bimetallic oxalates,9c,d and a detailed study is ongoing. It would be worthwhile to study the magnetic structure of VI and to further understand its properties. Conclusion A series of new organically directed bimetallic sulfates have been prepared hydrothermally. The results reveal that not only the amine but also the introduced second metal with different radii and coordination number both affect the structural features of metal sulfate. This study provides an aid in the design and synthesis of inorganic materials with novel structure and properties. Acknowledgment. We thank the Natural Scientific Foundation Committee of Shanxi Province (No. 20041031) for generously supporting this study. Supporting Information Available: Synthetic conditions for six compounds and hydrogen-bonding interactions, cell parameters for the

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